One difficulty with known systems is the loss of efficiency which results from exposure to moisture and impurities. Water vapor is sufficient to destroy the phosphors over a relatively short period of time.
Another difficulty is the sensitivity of electroluminescent systems to temperature fluctuations caused by electrical currents. The phosphor is particularly susceptible to degredation when subjected to heat over an extended period and this can lead ultimately to a complete failure of the device.
Still another problem is the inability of most dielectrics to transmit and maintain a high dielectric constant.
In U.S. Pat. No. 3,238,407 Mary Jaffe describes a system which attempts to overcome this problem by utilizing an insulating layer of barium titanate and cyanoethyl cellulose (CEC) in powdered form. This composition is interposed between a phosphor layer and a conductive layer and it has the effect of providing enhanced brightness. However, it has since been discovered that CEC in electroluminescent cells diminishes the adhesive capabilities of the system.
To overcome this undesirable effect plasticizers have been added to CEC-containing matrixes. Typical of these are, for example, phthalate esters, cresyl diphenyl phosphate and diethylene glycol. However, the addition of said plasticizers has not been entirely successful because although they produce the desired effect they also diminish the intensity of emitted light and have an adverse effect on lamp longevity.
Other disadvantages associated with CEC are its high cost and its tendency to interfere with the oxidation of available zinc in the phosphor component as a result of which the luminescent properties of the system are greatly diminished.
Moreover, the commercial availability of CEC is so unreliable and its purity so questionable that it cannot be depended upon to provide a dielectric matrix of predictably constant properties.
In U.S. Pat. No. 3,389,286 Takahashi et al describes an electroluminescent device in which the electrodes are separated by a dielectric which contains cyanoethylated polyvinyl alcohol (CEPVA). This device is superior to dielectrics containing plasticizers alone and it is said to provide good transparency, high insulative value and low electrical loss. However, the addition of CEPVA results in the partial dissolution of the phosphor component and a diminution of light intensity.
Moreover, CEPVA is sensitive to cold flow under pressure and this is undesirable in systems where springloaded electrical contacts are employed.